x86/p2m: fix PoD accounting in guest_physmap_add_entry()

x86/p2m: fix PoD accounting in guest_physmap_add_entry()

The initial observation was that the mfn_valid() check comes too late:
Neither mfn_add() nor mfn_to_page() (let alone de-referencing the
result of the latter) are valid for MFNs failing this check. Move it up
and - noticing that there's no caller doing so - also add an assertion
that this should never produce "false" here.

In turn this would have meant that the "else" to that if() could now go
away, which didn't seem right at all. And indeed, considering callers
like memory_exchange() or various grant table functions, the PoD
accounting should have been outside of that if() from the very
beginning.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>